1. Field of the Invention
This invention is related to the field of integrated circuits including integrated memories such as static random access memory (SRAM) and, more particularly, to supplying power to such integrated circuits.
2. Description of the Related Art
As the number of transistors included on a single integrated circuit “chip” has increased and as the operating frequency of the integrated circuits has increased, the management of power consumed by an integrated circuit has continued to increase in importance. If power consumption is not managed, meeting the thermal requirements of the integrated circuit (e.g. providing components required to adequately cool the integrated circuit during operation to remain within thermal limits of the integrated circuit) may be overly costly or even infeasible. Additionally, in some applications such as battery powered devices, managing power consumption in an integrated circuit may be key to providing acceptable battery life.
Power consumption in an integrated circuit is related to the supply voltage provided to the integrated circuit. For example, many digital logic circuits represent a binary one and a binary zero as the supply voltage and ground voltage, respectively (or vice versa). As digital logic evaluates during operation, signals frequently transition fully from one voltage to the other. Thus, the power consumed in an integrated circuit is dependent on the magnitude of the supply voltage relative to the ground voltage. Reducing the supply voltage generally leads to reduced power consumption. However, there are limits to the amount by which the supply voltage may be reduced.
One limit to the reduction of supply voltage that is experienced in integrated circuits that integrate memories (such as SRAM) is related to the robustness of the memory. As supply voltage decreases below a certain voltage, the ability to reliably read and write the memory decreases. The reduced reliability may have several sources. The resistances of some devices in the memory (e.g. the pass gate transistors that couple bit lines to memory cells in an SRAM) may change as the supply voltage falls. The changed resistance may impact the ability to overdrive the memory cell for a write or to discharge the bit line for a read. Additionally, in some designs, the transistors in the memory are high threshold voltage (high VT) transistors. That is, the threshold voltage at which the transistors activate (or “turn on” . . . i.e. actively conduct current) is higher than other transistors in the integrated circuit. The threshold voltage of such transistors does not scale well with supply voltage. Accordingly, the “trip point” (the point at which a write to a memory cell occurs) as a percentage of the supply voltage worsens as the supply voltage is decreased. As an example, in one current integrated circuit manufacturing process, a supply voltage below about 0.9 volts results in reduced ability to write the memory reliably. Similarly, the ability to quickly and/or reliably read the memory decreases. Accordingly, the supply voltage at which the robustness of the memory begins to be impacted has served as a floor to reducing the supply voltage to an integrated circuit that includes memory.